An accidental change in the attitude of a satellite may be produced following various types of failure or clumsy manoeuvres. Among others, the following may be noted:
equipment failures, such as failure of the drive system of the solar array, seizure of the momentum wheels, earth sensor optical fault or thruster sticking open; PA1 electrical failures, essentially in the inertia unit, or also variations in the power available causing a temporary failure of the attitude control servo-loops; PA1 design limitations in the satellite control unit, for example cases of the perturbation of the earth sensors by the passage of the sun or the moon through their field of view or also if servo-loops interact in unforeseen manner; PA1 errors of manipulation due to programming mistakes in the on-board software, or erroneous commands received from the ground, under automatic or manual control. PA1 automatic reconfiguration mode (ARM); PA1 emergency sun reacquisition (ESR). PA1 the reaction time of fifteen minutes is usually too short to enable the operator to react and can scarcely be prolonged without using up excessively the solar energy reserves; PA1 the emergency sun pointing corresponds to achievement of maximum security but causes the use of micro-thrusters in a control-loop consuming up to several kilograms of propellant. Also, failure of the spare micro-thrusters used in ESR mode (leakage or accidental prolonged operation) may completely compromise the stability in ESR mode. The time for earth repointing from the ESR mode is too long. This point is especially true for the new generation of telecommunication satellites (Inmarsat 2, ECS-A) in which it is necessary to limit the possible loss of communication to a maximum of one-and-a-half hours. This constraint is now also a requirement for satellites already in orbit. PA1 phase A of the procedure enables bringing back the satellite from an ESR mode to an intermediary attitude in which the pitch axis of the satellite moves within a reduced range of values around the earth direction, with reduced nutation and angular rates. This phase is accomplished through determination and control of pitch angular rate and position of the satellite relative to the sun direction. Two embodiments for this phase A will be detailed hereunder. PA1 phase B corresponds to the fast recovery phase strictly speaking, and consists in bringing the satellite from its reduced nutation and angular rates into three axes earthwards stabilization. This is obtained through a succession of steps intended at first progressively damping the nutation rate, and then precisely aligning the pitch axis of the satellite. This phase is differently carried out according to whether the satellite is momentum bias stabilized, or zero momentum stabilized. Moreover, in the embodiment of phase B corresponding to momentum bias satellites, the steps correspond to a better stabilization when following up the procedure. This enables selectively entering phase B at an intermediary step of phase B for earth pointing recovery from slight attitude modifications following non important contingencies. PA1 (i) an initialization step comprising deactivating the servo loop of said roll servo control means, connecting all said sun and earth sensors and/or gyrometers, and assuming open-loop control of attitude using said attitude correction means; PA1 (ii) a pitch pre-stabilization step comprising placing the satellite in dual spin configuration, with counterrotating body, responding to signals from said sun and earth sensors to determine pitch rate, and controlling the pitch rate using said attitude correction means; PA1 (iii) a pitch recovery step comprising bringing the pitch rate within the operating limits of said pitch servo control means by earth capture in pitch with single axis control and; PA1 (iv) a residual nutation damping step comprising exerting transverse roll/yaw torque by action on said momentum wheel and/or thrusters until earth capture by said roll servo control means. PA1 a solar array servo control step in which the solar arrays are pointed and locked sunwards using sun sensors mounted on said solar panels; PA1 a solar panel skewing step in which the bearing and power transfer assembly is skewed relative to said solar panels by an angle related to that between the sun and earth directions seen from the satellite at the local time of the manoeuvre; PA1 a satellite attitude determination step in which the satellite attitude (angular rate and position of the pitch axis) is determined by reference to the cyclic variation of signals from said earth sensor during roll movement of the satellite. PA1 The steps for this second embodiment of phase A may be summed up as follows: PA1 a star detection step in which stars are detected using said star tracking means; PA1 a satellite attitude determination step in which the satellite attitude is determined by reference to the location of said stars; PA1 a roll rate arresting step with wheel spin up and/or gyrometer initialization at a time when the pitch axis is in a position favourable to the fast earth recovery of phase B as described above.
This type of failure is likely to perturb the operation of the satellite seriously, or even interrupt it, and it is only operational under strict conditions of orientation. This is especially the case for geo-stationary telecommunication satellites stabilized on three axes in earth-pointing.